But scientists with the National Oceanic and Atmospheric Administration are working to prevent outbreaks by tracking when and where red tides will happen next.

The toxic algae sleep nestled in the muck at the bottom of Puget Sound. A team of NOAA scientists based in Seattle was recently out looking for the algae so they can predict where and how big the red tides might be in the spring and summer — when the algae wake up and start to infect shellfish.

It's hard to predict where these blooms will occur because winds and currents move the clouds of algae around. So the scientists target areas where blooms have occurred in the past.

They've found high levels of algal cysts in areas that later had high levels of poisoned shellfish. The toxins don't go away when shellfish are cooked or frozen.

No one has died from paralytic shellfish poisoning in Washington state since the 1940s. But every year the state's Department of Health manually tests thousands of clams, oysters and mussels for toxin levels. And every year some shellfish beds are closed to harvesting.

Mapping the algae at the bottom of Puget Sound won't replace manually testing the shellfish, but the NOAA team wants to make the Department of Health's job easier.

"We want to be able to say when and where there is a really strong chance of a bloom occurring so that the shellfish growers and the health managers are able to put in place some strategies to try and prevent, or reduce and limit, the impacts that these blooms can have," says Stephanie Moore, a NOAA scientist.

Moore says scientists have a lot to learn about what triggers these red tides. One thing's certain though: The algae love warm sunshine-filled water. Researchers say they could see more blooms as water temperatures rise.

Ashley Ahearn is a reporter for NPR member station KUOW in Seattle.

Copyright 2012 National Public Radio. To see more, visit http://www.npr.org/.

Transcript

MELISSA BLOCK, HOST:

From NPR News, this ALL THINGS CONSIDERED. I'm Melissa Block.

It's a trip into Puget Sound in search of red tide. That's the name for huge blooms of naturally occurring toxic algae. Each spring and summer, they can poison shellfish. People who eat infected clams or oysters can become sick with what's called paralytic shellfish poisoning. One challenge for public health is figuring out when or where red tide will occur. Ashley Ahearn of member station KUOW hopped on a boat with some scientists who are trying to answer that.

ASHLEY AHEARN, BYLINE: Stephanie Moore stands on the deck of the Clifford Barnes, a 65-foot research vessel heading out of Seattle Harbor. She's a woman on a mission.

DR. STEPHANIE MOORE: We are hunting toxic algal cysts. Yeah.

(SOUNDBITE OF LAUGHTER)

AHEARN: Moore is with the National Oceanic and Atmospheric Administration and an expert on harmful algal blooms. She and her team are going out to find these toxic algae where they sleep, nestled in the muck at the bottom of Puget Sound. That way, they can predict where and how big the red tides might be in the spring and summer when the algae wake up and start to infect shellfish. Once we're offshore, the boat stops, and the team lowers a large metal cylinder into the water. When it gets to the bottom, it basically works like a drinking straw in a thick milkshake, sucking in a 6-inch tube of muck.

MOORE: What's our depth, Jules?

JULIE MASURA: 196.

MOORE: 196. OK. Here we go. What we're sampling in the sediment are cysts of the home for algal species. So these guys just have this dormant resting cyst stage that they use to overwinter, and there are cues that, you know, that tell them when it's time to germinate and to swim to the surface and to start growing.

AHEARN: The algae Moore is talking about is called Alexandrium. It's a sort of superbug: half plant, half animal. This single-celled organism has a whiplike tail that propels it through the water, and it can photosynthesize. So once it swims from the depths up to the sunlight in the warmer weather, it starts to divide and divide and divide until it's formed what's called a bloom. Winds and currents move these clouds of algae around, so it's hard to predict where they'll show up next. This team of scientists is trying to solve that problem. The ship's crane raises a tube of muck from more than 600 feet below.

MASURA: And we have mud.

AHEARN: From here, Julie Masura shaves off layers of mud from the tube, removes excess water and prepares it for the lab. Masura is a researcher at the University of Washington in Tacoma. She spent a lot of time looking at these super beasts under the microscope.

MASURA: We stain them, and they fluoresce, kind of like green jellybeans. And so I count them.

AHEARN: When the team goes out to collect samples, they target areas where blooms have occurred in the past. They've found high levels of cysts in areas that later had high levels of poisoned shellfish. No one has died from paralytic shellfish poisoning in Washington since the 1940s, but every year, the state's Department of Health manually tests thousands of clams, oysters and mussels for toxin levels. And every year, some shellfish beds are closed to harvesting. These toxins don't go away when shellfish are cooked or frozen.

Mapping the algae at the bottom of Puget Sound won't replace manually testing the shellfish, but Moore and her team want to make the Department of Health's job easier.

MOORE: We want to be able to say when and where there's a really strong chance of a bloom occurring so that the shellfish growers and the health managers are able to reduce and limit the impacts that these blooms can have.

AHEARN: Moore says scientists have a lot to learn about what triggers these red tides. One thing is certain, though, these creatures love warm, sunshine-filled water, and researchers say they could see more blooms as water temperatures rise. For NPR News, I'm Ashley Ahearn in Seattle. Transcript provided by NPR, Copyright National Public Radio.